1) In Homer several words end in "-then", refering to motion away from a place. Some genitives also end in "-then", such as "emethen". I know that the genitive in Attic Greek includes the ablative. So is the "-then" ending a "true" ablative?

2) What is the -de suffix on words (oikade, athenazde etc) , grammatically speaking? It indicates a kind of motion - motion towards. Is it a kind of case? Or is it something more like agglutination?

The adverbial endings ([face=SPIonic]-fi, -qen, -de, -qi[/face]) placed after a word may illustrate the process by which an inflection first comes to be. Some authorities think that all case endings began like this.

[face=SPIonic]-fi[/face] : this suffix is used by Homer as an instrumental, ablative, locative, and dative.

[face=SPIonic]-de[/face] : as you say, originally, 'motion towards'. This is the so-called 'allative suffix' use. Note that the accusative case carries an implicit sense of 'motion towards'. As this sense was lost, the -de suffix occurs more frequently.

[face=SPIonic]-qen[/face] : as you say, originally an ablatival case ending meaning, 'motion from'.

These 3 suffixes are attested in Mycenaean (as derived from Linear B) with the uses described above.

The dative subsumes the locative and instrumental; the genitive subsumes the ablative.

logoi becomes logw(i) because this an unequal diphthong - the o is longer than the i. This is a "long diphthong" which apparently occurs in IE and usually resovles itself into and ordinary diphthong or just the stronger vowel.

Thucydides wrote:logoi becomes logw(i) because this an unequal diphthong - the o is longer than the i. This is a "long diphthong" which apparently occurs in IE and usually resovles itself into and ordinary diphthong or just the stronger vowel.

It is fairly clear that the 'o' in [face=SPIonic]lo/gw|[/face] and the 'o' in [face=SPIonic]moi[/face] are different. In the latter the 'o' remains short. In the former it becomes long most probably by contraction with either *-ei or *-ai (IE dative singular case endings; see Smyth, Palmer, ktl). This gives rise to the IE suffix *-oi (with a long 'o') whence Greek [face=SPIonic]wi[/face].

I'm not sure it's right to say that '[face=SPIonic]lo/goi[/face]becomes [face=SPIonic]lo/gw|[/face]because this is an unequal diphthong..'. Rather, because of the aforementioned contraction, the word - its long diphthong already intact - was inherited by Greek with the [face=SPIonic]wi[/face] ending.

...indeed I do. And to go further: how did cases originate anyway? I'd assume there are several theories. It seems to me that cases are more numerous in older and more synthetic languages.

Most cases are used for relationships - dative - to, ablative - away from etc. In this case (!) it seems that cases serve pretty much the same role as prepositions. But what about genitive, nominative and accusative?

Accusative (or so I have heard/misunderstood...) evolved out of the idea of motion towards a thing - so another relationship.

Nominative is presumably the "pure" (uninflected) form of the word.

The idea of possesion of the genitive case seems odd. Did it perhaps originally indicate a relationship like "contained by" or "within"?

But if the subject/object uses of cases evolved out of inflections that originally only indicated relationships, what was used to indicate the subject object situation before this?

(This may be very very wrong... I'm only hypothesising from my moderate knowledge of languages...)

...indeed I do. And to go further: how did cases originate anyway? I'd assume there are several theories. It seems to me that cases are more numerous in older and more synthetic languages.

Oh, gosh. Look at Finnish! 15+ cases, depending on how you count. Most of them match pretty nicely to prepositions with motion or location.

Accusative (or so I have heard/misunderstood...) evolved out of the idea of motion towards a thing - so another relationship.

I suspect not. In some languages the direct object is marked with a preposition, one quite different from prepositions for motion.

(This may be very very wrong... I'm only hypothesising from my moderate knowledge of languages...)

There's a technical term for what happens when words go from being parts of vocabulary to being for grammar only. It's "grammaticalization." Google on that for articles galore.

For example, in English the verb "will" has lost most of its use meaning "to want" and is used by far the majority of the time for the future tense. The future tense in several Romance languages come from the infinitive + a form of habere, then they all mashed together into a conjugation.

I'm not sure we can say where each Indo-European case came from (doubtless many have speculated freely), but I think it's safe to say one or two probably started out as postpositions (rather than prepositions) that merged with the noun stem. I think genitives have a tendency to evolve out of adjective derivations.